1. Field of the Invention
This invention relates to an electroluminescent element and particularly to a dispersion type electroluminescent element in which the electroluminescent layer existing between the electrode plates comprises an organic dielectric having a high dielectric constant and an electroluminescent phosphor.
2. Description of the Prior Art
The electroluminescent element is a planar light source capable of emitting luminescence of various colors with low power consumption. Particularly, a powdery dispersion type electroluminescent element can be readily prepared at a low cost with a relatively large area, and thus application of a powdery dispersion-type electroluminescent element to a display device, a display and a planar television, etc. is expected. However, the dispersion-type electroluminescent element has a poor brightness and a short life, and thus has not been practically used.
ZnS is a practical electroluminescent phosphor when used in the electroluminescent layer of an electroluminescent element. ZnS has such properties that (a) the brightness depends greatly upon the electric field and (b) the brightness increases substantially in proportion to the driving frequency, but the half-life of brightness decreases in inverse proportion to the driving frequency. Thus, in the production of an electroluminescent element having a long half-life of brightness from ZnS particles, it is necessary to obtain practical brightness with a low driving frequency. One means is by uniformly dispersing ZnS particles into a dielectric having a high dielectric constant in an electroluminescent layer existing between electrode plates and increasing the electric field application to the phosphor particles in the electroluminescent layer to a maximum. Since the electroluminescent layer is in a film state, an organic dielectric that can be readily made into a film must be used as the dielectric having a high dielectric constant. Those organic dielectrics known to have a high dielectric constant, which can be readily made into a film, include cyanoethylated cellulose, cyanoethylated polyvinyl alcohol, etc. which have a dielectric constant of 12 to 21.
Organic dielectrics having a dielectric constant of 30 or higher are in a liquid state at room temperature. When electroluminescent phosphor particles are dispersed into a liquid organic dielectric to prepare an electroluminescent layer, a practical brightness can be obtained in the initial period, but the phosphor particles migrate and undergo condensation while the layer is subjected to emission of electroluminescence under application of an electric field, and the electroluminescent surface is disturbed resulting in a failure to display. To improve such phenomena, it was tried to use a mixture of a liquid organic dielectric having a high dielectric constant and cyanoethylated cellulose or cyanoethylated polyvinyl alcohol having a dielectric of 12 to 21 in the electroluminescent layer, but it was found that the dielectric constant of such a mixture was 20-30% lower than that of the original liquid organic dielectric.